The research proposed centers around the long-term objective of understanding uterine mononuclear leukocyte function, in both the non-pregnant and pregnant states. The experiments are based on several propositions: 1) that the uterus, a mucosal organ, must maintain a cellular armamentarium for defense; 2) that the unique function of the uterus demands changes during early pregnancy to prevent dangerous cells from accessing the conceptus; 3) that certain local mononuclear cells may have paracrine effects beneficial to pregnancy;and 4) that pregnancy failure can result from altered distribution or function of mononuclear cell populations. To address these postulates, the precise identification of specific uterine mononuclear cells is essential, but a vital issue is where the different cells are in the tissue, especially related to the implanting conceptus. Although much information has been gained on lymphoid cells in human pregnancy tissues, the crucial peri-implantation period can only be studied in animal models. Thus, in situ immunohistochemistry with highly specific, mostly monoclonal antibodies and cell tracing techniques will be used in rats and mice as models of interstitial implantation. The specific aims are: 1) to identify the presence, locations, and activation states of mononuclear cells, e.g., T cell subsets (including T alpha/beta vs. T gamma/delta), macrophages, and natural killer (NK) cells, in the nonpregnant uterus and determine changes occurring with implantation; lineage designations will be sought for pregnancy-associated cells -- suppressor cells and granulated metrial gland cells; the importance of NK lineage cells will be ascertained in animals depleted of NK cells. 2) to trace lymphoid cells into the uterus using fluorescent labels, identify the sites of entry by both in vitro adhesion assays and in situ staining for endothelial migration-associated ligands, and determine differences in decidualized tissue that could explain exclusion of some cells and recruitment of others; the features important in modulating cell influx will be analyzed using pseudopregnant animals with intrauterine treatments and uteri containing mini-osmotic pumps to dissect specific components of decidualization, especially prostaglandins. 3) to determine the types, numbers, and distribution of mononuclear cells in uteri of mice experiencing apparent immune-associated abortion; these will be compared with sites from the t(w73) mutant with similarly timed, but physiologically mediated pregnancy loss. Endometrial leukocytes undoubtedly have a major role in protection of this mucosal surface, but there is increasing evidence that they can have a negative impact on pregnancy, causing infertility or pregnancy loss. Conversely, recent experiments suggest that some leukocytes have positive influences on implantation and placental growth. The proposed research will provide essential information towards understanding how these paradoxical effects can occur, how they may be regulated, and the paracrine interactions important to uterine function.